Isolated Amaranthus agglutinin gene and uses thereof

ABSTRACT

The present invention provides the cloning of intron-containing  Amaranthus caudatus  agglutinin (ACA) gene from  Amaranthus caudatus  and the coding region gene sequence encoding the ACA protein. The said gene sequence of the ACA gene coding region was ligated with highly efficient and stable microbial expression vectors to produce the ACA protein using microbe largely. High expression of the ACA gene can make the transgenic plants have antiaphid activity and improve the nutrition quality thereof. The ACA gene has potential application value in plant anti-insect gene engineering and quality improvement.

FIELD OF THE INVENTION

This invention relates to the field of plant gene engineering for insect-resistance and biochemistry. Particularly, it relates to a newly cloned Amaranthus caudatus agglutinin (ACA) gene and the recombinant plasmids for conducting the expression of ACA gene in plants. Transgenic plants having aphid-resistance can be obtained from the transformation of plants with the said recombinant plasmids. Meanwhile, ACA protein could be produced using microbes harboring a high efficient and stable microbe expression vector for ACA gene.

BACKGROUND OF THE INVENTION

Amaranthus caudatus agglutinin (ACA) is a kind of phytoagglutinin (plant lectin) present in the seeds of Amaranthus caudatus having high nutritive value originated in South America continent [Vietmeyer, N. et al (1986), Science 232:1379–1384] and is mainly as a storage protein providing nutrition during the seed germination. ACA is abundantly synthesized in seeds during the formation of endosperm, but little in vegetative organs. The amino acid sequence of ACA protein has been reported in GenBank (Accession No. g2781234). Rahbe Y. et al carried out in vitro feeding pea aphids (Acyrthosiphon pisum) with different purified plant lectin proteins and the results showed that ACA gave the very high inhibition or toxic effect for tested aphids [Rahbe Y, Sauvion N, Febvay G et al 1995, Entomol Exp Appl, 76:143–155]. It is reported that this agglutinin plays an important role in specific recognition of tumor cells, histochemical identification and early diagnosis of tumors [Boland, C. et al, (1991), Cancer Res. 51:657–665]. As the function of plant agglutinins being further elucidated and potential application values thereof in medicine and biology being understood, it appears necessary to clone the ACA gene for studying and applying the agglutinin. Though the aphid-resistant activity of ACA has been revealed in the insect-resistant test in vitro as described, this activity has not been verified in transgenic plants expressing ACA gene. Moreover, ACA protein as a protein being rich in essential amino acid residues may play an important role in improving quality of crop. High level expression of ACA protein obtained in vitro conveniently could be a sufficient supply of this protein for identification of some tumor diseases.

BRIEF DESCRIPTIONS OF THE INVENTION

Based on the above situation, the purpose of this invention is to provide a novel structural gene of ACA protein, its recombinant plasmid and recombinant plasmid expressed in microbe and plants.

In order to obtain the structural gene of ACA, the structural gene of ACA was first obtained by polymerase chain reaction (PCR) using total DNA from A. caudatus as template. The gene consists of 2628 bp (SEQ ID NO:1) including an intron of 1716 bp and two exons of 212 bp (SEQ ID NO:2) and 700 bp (SEQ ID NO:3) respectively. High sequence homology between the amino acid sequence (SEQ ID NO:4) deduced from the nucleotide sequence thereof and the reported amino acid sequence of ACA protein reported by transue et al[Nature Structural Biology, 1997, 4(10):779–783] exists. A recombinant plasmid which can drive the ACA gene constitutively expressed in plants was constructed. The advantage thereof is that the recombinant plasmid can express the ACA protein in all tissues of the resulted transgenic plants. For expression of the ACA gene in particular tissue, a tissue-specific promoter could be used to make the ACA protein specifically be synthesized in the target tissue of transgenic plants. This type of expression of ACA protein could make the transgenic plants resistant to aphid efficiently. At the same time, since ACA protein is rich in essential amino acids, its high expression in transgenic plants could improve the nutritive value of the plants. An efficient and stable expression vector is constructed and abundant ACA protein could be synthesized in recombinant microbes, the protein could have certain application in tumor-detection.

The ACA gene is firstly cloned by this invention, this makes it possible to produce ACA in large quantity and to apply this agglutinin in agriculture and medicine.

In order to achieve the above purposes, the particular procedures leading to this invention were as follows: A pair of PCR primers were designed and synthesized according to the cDNA sequence of AmAl protein of Amaranthus hypochondriacus [Raina A et al. (1992). PNAS, Vol(89):11774–11778], which is highly homologous in amino acid sequence with ACA protein. ACA structural gene was amplified by PCR in the presence of genomic DNA extracted from seeds or leaves of A. caudatus and cloned. The purified PCR product was ligated with pUC18 vector and then transformed into E. coli DH5α to obtain the recombinant plasmid pACA containing ACA structural gene and E. coli DH5α transformant thereof. The transformant (pACA/DH5α) named pACAg/DH5α. was deposited in the China General Microbiological Culture Collection Center with accession No. CGMCC NO.0438. A pair of PCR primers were designed according to the sequence of the recombinant plasmid pACA which was used as a template, the DNA fragment in the coding region of cDNA of ACA was amplified by reverse PCR, and then transformed into E. coli DH5α after self-ligation to obtain the recombinant plasmid pACAc and the E. coli DH5α transformant thereof. DNA fragments of the cDNA and ACA structural gene were isolated from recombinant plasmid pACAc and pACA, and ligated with plant constitutive expression vector pBin438 respectively and then transformed into E. coli DH5α to obtain recombinant plasmids pBACAc and pBACA that can express constitutively in plant and E. coli DH5α transformant. Agrobacterium tumefaciens LBA4404 was transformed with recombinant plasmid pBACAc and PBACA extracted from the above transformants to obtain A. tumefacieus transformants pBACAc/LBA4404 and pBACA/LBA4404 respectively. At the same time, the DNA fragment of the ACA structural gene was isolated from the recombinant plasmid pACAc and ligated with E. coli expression vector pET30a(+), then transformed into E. coli BL21(DE3) to obtain a recombinant plasmid pEACAc that can express in E. coli and E. coli transformant pEACAc/BL21 (DE3). The E. coli transformant pEACAc/BL21(DE3) can express a specific ACA protein upon induction.

BRIEF DESCRIPTION OF THE DRAWINGS

The attached drawings are presented to further illustrate this invention in detail.

FIG. 1: Construction of a recombinant plasmid pACA of the ACA gene (Amp: ampicillin resistance)

FIG. 2: Total length nucleotide sequence as SEQ ID NO:1 of the ACA gene, letters in lower case represent the nucleotide sequence of the intron.

FIG. 3: Sketch map of the ACA gene structure, numbers on the map indicate the size of corresponding region in base pairs.

FIG. 4: Homologous comparison of amino acid sequence of the ACA protein deduced from the nucleotide sequence of ACA gene with the amino acid sequence of the ACA protein reported (ACA.PRO: amino acid sequence of the ACA protein (SEQ ID NO:5); ACAc.PRO: Amino acid sequence deduced from the cloned ACA gene of the invention (SEQ ID NO:4); amino acids in boxes indicate the differences between the two sequences)

FIG. 5: Construction of plant expression vector PBACA for ACA gene (NPTII: Neomycin phosphotransferase gene; 35S: CaMV 35S promoter with double enhancer sequences; NOS: Transcription terminator sequence of nopaline synthase gene; LB: T-DNA left border of Ti plasmid; RB: T-DNA right border sequence of Ti plasmid)

FIG. 6: Construction of plant expression vector pBACAc for ACA gene coding region (Kl: Kienow DNA polymerase)

FIG. 7: SDS-PAGE pattern of E. coli expression products of the ACA protein [M: Protein molecular weight standard; 1: pEACAc/BL21(DE3) 2: pET30a/BL21(DE3); Arrowhead points the position of the expressed target protein (ACA)]

DETAILED DESCRIPTION OF THE PREFERABLE EXAMPLES

Further detailed description of this invention is presented via particular examples and the attached figures.

EXAMPLES

1. Isolation of Total DNA from Amaranthus Caudatus Plants

Seeds of Amaranthus caudatus were kindly provided by Regional Plant Introduction Station of USDA, IOWA State University. The procedure of DNA isolation was referred to Paterson A H, Brubaker C L, Wendel J F et al [Plant Mol. Biol. Reporter, 1993, 11(2): 122–127] with some modifications. Two hundreds mg of leaves from Amaranthus caudatus plants or seeds that is maturing were ground into powder in liquid nitrogen and 1 ml of DNA extraction buffer [0.35 M glucose, 0.1 M Tris-HCl (PH8.0), 0.005 M Na2-EDTA (PH8.0), 2% (W/V) polyvinyl-pyrolidone (PVP40), 0.1% (W/V) diethylithiocabarmic acid (DIECA), 0.2% (W/V) mercaptoethanol, 0.5% Triton-X100] prechilled with ice was added. After mixed thoroughly, the suspension was centrifuged at 4° C., 2700×g for 10 mins to remove the supernatant. To the resulted pellet, 500 μl of nuclear lysis buffer [0.1 M Tris-HCl, PH8.0, 1.4 M NaCl, 0.02 M Na2-EDTA (PH8.0), 2% (W/V) CTAB (hexadecy1triammonium bromide), 2% (W/V) PVP, 0.1% (W/V) DIECA, 0.2% (W/V) mercaptoethanol, 0.5% Triton-X100] was added for re-suspension of the pellet at 65 ° C. for 30 mins. The resuspension was extracted with equal volume of chloroform-isoamyl alcohol (24:1) and to the resulted supernatant (the up-phase) after centrifugation at 10000 rpm, 0.6 vol of pre-cold isopropanol was added to precipitate DNA. After centrifugation at 12000 rpm, the precipitate was washed with 70% ethanol, dried at room temperature and dissolved in 50 μl sterilized water and stored at −20° C.

2. PCR Amplification of the ACA Gene

Twenty μl of a PCR system consisted of 1 μl of about 50 ng total DNA from Amaranthus caudatus, 2 μl 10×PCR buffer, 2 μl dNTP (the final concentration for each NTP is 10 mmol/L), 2 μl each of the two primers (final concentration is 10 pmol/μl), Taq Plus I (2.5 Units), the rest volume was made up with deionized water. The reaction system was covered with proper amount of mineral oil to prevent evaporation.

5′ primer sequence: 5′-GGA AGA TCT ACC ATG GCG GGA TTA CCA GTG-3′ 3′ primer sequence: 5′-AGC GTC GAC TTA GTT GTT GGA TCC CAA TTC-3′ The reaction condition is: pre-denaturing at 94° C. for 3 mins, and then 94° C. 1 min, 49° C. for 1 min, 72° C. 1 min 30 sec, with 30 cycles, an elongation of 10 mins at 72° C., was preformed after the 30 cycles reaction. The PCR products were separated by electrophoresis and two DNA fragments of 2.5 kb and 0.9 kb were isolated. Recovery of the two fragments was performed using the DNA recovery kit available from Shanghai Huashun Ltd and dissolved in 30 μl sterilized water. 3. Cloning and Sequencing of the ACA Gene

Methods for DNA clone and sequencing referred to <<Molecular Cloning, A Laboratory Manual>> edited by Sambrook J. et al., Cold Spring Harbor Laboratory Press, 1993. 10 μl of the recovered PCR product was ligated with 1 μl of pUC18-T vector in 20 μl of ligation system comprising 2 μl of 10×ligation buffer, 10 μl of PCR product, 1 μl of vector, 1 μl of T4 DNA ligase and 6 μl of water. The ligation reaction was performed at 14° C.˜16° C. for 12 hours. E.coli DH5α competent cells was transformed with 8 μl of the ligation mixture and then spreaded onto the LB solid medium containing 50 mg/L Amp (ampicillin). The positive colonies were selected using blue-white colony screening method. The plasmids of selected positive colonies were extracted and further identified by restriction enzyme digestion and sequencing analysis using Pharmacia T7 DNA Sequencing Kit. The results confirmed the insertion of the 2.6 kb ACA gene. The construction process of the recombinant plasmid is shown in FIG. 1, named as pACA. The complete nucleotide sequence of the ACA gene was analyzed on an ABI 377 DNA sequencer and the result is shown in FIG. 2. The result from sequence analysis indicate that the ACA gene having the DNA sequence of SEQ ID NO:1 is consisted of 2628 bp, containing an intron of 1716 bp and two exons of 212 bp (SEQ ID NO:2) and 700 bp (SEQ ID NO:3) respectively. The structure of the ACA gene is shown in claim 1 and diagramed in FIG. 3.

4. Coding Sequence of the ACA Gene Obtained by Reverse PCR

Based on the sequence of the ACA gene (SEQ ID NO:1), two primers complemented with the exons adjacent to the intron respectively were designed and synthesized. The sequences of the two primers were as followes:

5′ end sequence: 5′-GTG GTC TCC CAA TCA TTA TTG-3′ 3′ end sequence: 5′-CTA ACC AAA TAT TTG TTA GTG-3′

Reverse PCR was performed in a total volume of 20 μl containing 1 μl dNTP (10 mmol/L each); 1 μl of each primer (25 pmol/μl); 1 μl of the diluted pACA solution as template (pACA extracted from E. coli transformant by alkaline lysis method was diluted 500 fold); 2 μl of 10×PCR buffer, 0.4 μl of pfu Taq DNA polymerase (5unit/μl), the final volume was made up with sterilized water. The reaction condition is: pre-denaturing at 94° C. for 3 mins, and then 94° C. 50 sec, 58° C. for 50 sec, 72° C. 2 mins 30 sec, with 30 cycles, an elongation of 10 mins at 72° C. was preformed after the 30 cycles reaction. The PCR product showed a 3.5 kb DNA in size on Agarose gel electrophoresis. The PCR product was purified and then phosphorylated by T4 polynucleotide kinase. After precipitated with ethanol, the DNA pellet was dissolved in 20 μl sterilized water.

5. Cloning and Sequencing of the Coding Region of the ACA Gene

2 μl of 10×ligation buffer, 1 μl T4-DNA ligase (1 Unit/μl) and 7 μl of sterilized water was added to 10 μl of the above purified product, the final volume was 20 μl. The reaction mixture was incubated at 14˜16° C. for 12 hours. Competent cells of E. coli DH5α was transformed with 8 μl of the ligation mixture and spreaded onto the LB solid medium containing 50 mg/L Amp. Plasmids in the selected transformants were isolated using alkaline lysis method and a 900 bp fragment was produced after digestion with Nco I and Sal I as expected. The recombinant plasmid is designated as pACAc. The coding sequence in pACAc was verified by sequence analysis on an ABI 377 DNA sequencer. The coding sequence of ACA gene (SEQ ID NO:2 and SEQ ID NO:3) in pACAc is shows in FIG. 2 (represented by capital letters). Comparison of the amino acid sequence (SEQ ID NO:4) encoded by ACA gene with that of the published ACA sequence is shown in FIG. 4.

6. Construction of Plant Expression Vector for ACA Gene Coding Sequence and Preparation of Recombinant Agrobacterium Tumefacieus.

Plasmid pACAc was digested by Bgl II and Sal I. The fragment of the ACA structural gene was recovered and then ligated with the fragment of the vector pBin438 digested by BamH I and Sal I [Li T-Y, et al, Science in China (B series), (1994) 24: 276–282] to construct the recombinant plasmid pBACAc, as outlined in FIG. 5. After transformation of E. coli competent cells with the ligation mixture, the cells were spreaded onto the LB solid medium containing 50 mg/L Kan(kanamycin). The recombinant plasmid was identified by restriction enzyme digestion and PCR method. Purified pBACAc using alkaline lysis method was transformed into competent cells of Agrobacterium tumefacieus LBA4404, the cells were spreaded onto the YEP solid medium containing 50 μg/μl Kan, 50 μg/μl Rif(rifampin), 50 μg/μl Str(streptomycin). The selected single colony was analyzed by restriction enzyme digestion and PCR detection to obtain the tranformant of Agrobacterium tumefacieus LBA4404 containing pBACAc.

7. Construction of Plant Expression Vector Containing the ACA Gene

The ACA gene fragment from pACA was digested by Nco I, filled in the cohesive ends with klenow DNA polymerase and then digested by Sal I to obtain a 2.6 kb of gene fragment. The isolated fragments were ligated with vector pBin438 fragment digested by BamH I and then filled in with klenow followed by digested with Sal I to form recombinant plasmid pBACA. The ligation mixture was transformed into competent cells of E. coli DH5α and the recombinant plasmids in the transformants selected were identified by restriction enzyme digestion and PCR detection. The preparation of Agrobacterium tumefacieus was as the same with that of described in above 6. Besides CaMV 35S promoter, tissue-specific promoters or other kind of promoters could be used to construct a plant expression vector with the ACAc or ACA gene. The construction process was shown in FIG. 6.

8. Construction of a Expression Vector for ACAC Gene and its Expression in E. coli.

The plasmid pACAc was digested by Nco I/Sal I. About 0.9 kb fragment was recover and then ligated with a expression vector pET30a (+) digested by the same enzymes. The recombinant plasmid named as pEACAc was used to transform E. coli BL21 (DE3) cells. Induced expression of the ACA gene was achieved based on the procedure provided by Qiagene Company[A handbook for high-level expression and purification of 6×His-Taged protein]. A single colony of pEACAc transformants was inoculated into liquid LB medium containing 50 μg/μl kamamycin and cultured at 37° C. with shaking at 250 rpm. When the bacteria grow to OD=0.2, isopropyl-β-D-thiogalactoside (IPTG) was added to a final concentration of 1 mmol/L. The culture was shaked continually in the same condition for 3h. Total protein of the bacterial cells was analyzed by SDS-PAGE as described by Laemmli UK. (Nature 227:680–685, 1970) using pre-stained protein marker from BioLab as protein MW standard. Results from SDS-PAGE showed that a 36 kD specific protein was produced in pEACAc transformant (FIG. 7, lane 1) while this protein is absent in E. coli BL 21 (DE3) cells (FIG. 7, Lane 2), indicating that the ACA protein was expressed in E. coli. The expression level of the ACA protein is estimated to be 35% of total bacterial proteins based on the calculation of gel-scanning. SDS-PAGE pattern was shown in FIG. 7.

Notes:

-   LB medium: 5 g/L NaCl; 10 g/L Trypton; 5 g/L Yeast extract; PH7.0;     12 g/L Agar; Autoclaved at 15 pound for 30 mins. -   YEP medium: 5 g/L NaCl; 10 g/L Trypton; 10 g/L Yeast extract; PH7.0;     12 g/L Agar, Autoclaved at 15 pound for 30 mins. 

1. An isolated gene having the nucleotide sequence of SEQ ID NO:
 1. 2. A recombinant plasmid comprising the nucleotide sequence of SEQ ID NO:
 1. 3. A recombinant plasmid comprising the nucleotide sequence of SEQ ID NO: 2 and the nucleotide sequence of SEQ ID NO:
 3. 4. A recombinant microbe comprising the recombinant plasmid of claim 2, wherein the recombinant microbe is Eschenchia coli DH5α deposited under the Accession number CGMCC
 0438. 5. A method of producing Amaranthus caudatus agglutinin protein comprising transforming a host cell with an expression vector comprising the nucleotide sequence of SEQ ID NO: 2 and the nucleotide sequence of SEQ ID NO: 3; and culturing the transformed host cell for the production of the protein.
 6. A transgenic plant comprising the recombinant plasmid of claim
 2. 7. A method of producing Amaranthus caudatus agglutinin protein comprising transforming a host cell with an expression vector comprising the nucleotide sequence of SEQ ID NO: 1; and culturing the transformed host cell for the production of the protein.
 8. A transgenic plant comprising the recombinant plasmid of claim
 3. 9. A host cell transformed with the recombinant plasmid of claim
 2. 10. A host cell transformed with the recombinant plasmid of claim
 3. 